The purpose of this study is to determine whether nutritional (cholecalciferol) or active vitamin D (calcitriol)supplementation improves vascular endothelial function in patients with stage IIIB and IV chronic kidney disease with vitamin D insufficiency or deficiency. The investigators hypothesize that the use of calcitriol supplementation will result in improved vascular endothelial function as compared to cholecalciferol supplementation.

Significant co-morbid conditions that lead the investigator to conclude that life expectancy is less than 1 year

Expected to undergo living related kidney transplant in next 6 months

Pregnant, breastfeeding, or unwilling to use adequate birth control

History of severe liver disease

Nephrotic range proteinuria (> 3.5 gm/day)

Use of active vitamin D analogs within 30 days of randomization

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Please refer to this study by its ClinicalTrials.gov identifier: NCT01384539